1. Field of the Invention
This invention relates to an optical disk apparatus for optically reading data from a disk of a recording medium and, more particularly, to an optical head comprising an optical system including a light source and a photodetector for detecting a light reflected from the disk.
2. Description of the Related Art
FIG. 7 through FIG. 9 of the accompanying drawings illustrate a structure of a conventional optical head. FIG. 7 is a schematic view showing a structure of the optical head employed in the optical disk apparatus shown in FIG. 7; FIG. 8, a sectional side elevation view of a light beam emitting device used in the optical head; and FIG. 9, a layout drawing showing optical components of the beam emitting device.
In the drawings, reference numeral 1 designates a disk as a recording medium; 2, a disk motor; 3, an optical base; 4, an objective lens drive unit; 5, an objective lens; 6, a reflecting mirror disposed at a part of the drive unit 4 and for guiding a light beam 9 emitted from a light beam emitting device 50 as will be described later. A condensing device 40 is composed of the objective lens drive unit 4, the objective lens 5 and the reflecting mirror 6. Numeral 7 designates a base for supporting the disk motor 2 and the optical base 3, etc.; 9, a light beam emitted from the light beam emitting device 50 as will be described later; 9a, an optical axis representing the center of the light beam 9; 20, a micro beam spot condensed by the objective lens 5 onto a data recording surface of the disk 1; 11, a semiconductor laser; 12, a collimator lens converting the light beam emitted from the semiconductor laser 11 into a parallel beam; and 13, a half prism. A light beam emitting device 50 is composed of the semiconductor laser 11, the collimator lens 12 and the half prism 13. Likewise, reference numeral 14 designates a semiconductor laser supporting base; 15, a cover; 16, a polarizing beam splitter; 17, a Foucault prism; 18 and 19, condensing lenses; 20, a split-light detector for detecting tracking errors; 21a and 21b, a split-light detector for detecting focusing errors; and 24, a .lambda./2 plate.
The principle operation of the conventional optical head will be described hereinbelow. As shown in FIG. 8, a light emitted from the semiconductor laser 11 is converted into a parallel beam by the collimator lens 12, and the parallel beam subsequently passes through the half prism 13 and is emitted from the light beam emitting device 50 as the light beam 9. Meanwhile, as shown in FIG. 7, the objective lens drive unit 4 is supported traversably in a radial direction (a direction of B) of the disk 1 by means of the slide bearing or the like (not shown) and is drive in the radial direction (direction B) by a linear motor (not shown), etc. The emitted light beam 9 is directed to the objective lens 5 by means of the reflecting mirror 6 mounted at one end of the objective lens drive unit 4 and is then focused into the micro beam spot 10 of about 1 to 2-micrometer diameter upon the data recording surface of the disk 1. Not fully described in the specification, the objective lens drive unit 4 corrects focusing errors and tracking errors relative to the data recording surface of the disk 1 by driving the objective lens 5 in the direction of the optical axis and in the radial direction (a direction of B) orthogonal to the surface of the disk 1.
The light beam reflected from the disk surface 1 is further reflected by the half-prism 13 by way of the objective lens 5 and the reflecting mirror 6 as illustrated in FIG. 9, and is separated into two directions by means of the polarizing beam splitter 16. The light beam 22 of the two-split light beams that is transmitted through the polarizing splitter 16 is converged onto the photodetector 20 by the condensing lens 19. The photo detector 20 converts the incident light beam into electric signals, and a tracking error is detected from a differential output of the signals. On the other hand, the light beam reflected by the polarizing beam splitter 16 is further divided into a light beam 23a and a light beam 23b by means of the Foucault prism 17, and also converged by the condensing lens 18 to be incident on the photo detectors 21a and 21b which are used for detecting focusing errors and disposed at the image forming point. The focusing error is detected from each differential output. The foregoing objective lens drive unit 4 drives the objective lens 5 in the direction of the optical axis and in the radial direction in response to the quantity of tracking errors and focusing errors to correct the focusing error and the tracking error. Here, data can be obtained from the disk 1 by adding signals output from the photo detector 20 and the photo detectors 21a and 21b.
In this conventional optical head, the height of the optical disk apparatus is dependent on the height of the optical head (H in FIG. 7). Specifically, the height of the optical head H is the sum of the height h1 between the surface of the disk (1) and the center of the reflecting mirror 6, or the height between the upper surface of the disk 1 and the center of the optical axis of the light beam 9 emitted from the light beam emitting device 50, the height h2 including the height between the center of the optical axis 9a of the light beam 9 and a center of the height of optical parts such as a collimator lens 12, or the bottom thickness of the optical base 3 or the like, and the height h3 of the thickness of the base 7.
With conventional optical head apparatus having thus mentioned structure, it is necessary to secure a space for depositing a non-illustrated objective lens holder or a drive circuit in the height h1 between the upper surface of the disk and the center of the reflecting mirror 6 in designing the objective lens drive device 4. Hence, there was a limit for reducing the size of the apparatus. Moreover, the height h2 is chiefly decided by the dimension of the parts used in the light beam emitting device (50), and therefore it was difficult to reduce the space of the height h2. In addition, the height h3 of the base also has a limit for reducing its dimension in view of ensuring a sufficient unit strength.